Fluid volume flows can be regulated, for example by means of fluid pumps with a variable delivery volume or by fluid pumps with a constant delivery volume and downstream control valves. As volume flow regulating valves, in general throttle-type distribution valves connected to a substantially constant fluid supply and referred to below as control valves are used, whose functional principle is that a volume flow admitted is throttled to a greater or lesser extent by a sharp-edged diaphragm of variable cross-section so that the volume flow emerging from the control varies. The pressure drop produced at the sharp-edged diaphragm increases quadratically with increasing volume flow and accordingly the pressure drop is small with a small volume flow but becomes rapidly larger as the volume flow increases. As a result, when the volume flow is small the regulation thereof reacts very sensitively to interfering factors (such as component tolerances, friction forces, control pressure tolerances etc). This problem increases with the size of the maximum volume flow for which a control valve is designed. When supplying cooling oil to clutches the additional problem arises that, particularly in cold weather, residual volume flow produces a damaging drag torque which can make synchronization in a downstream transmission more difficult, so that in such cases too the cooling oil supply to the clutch has to be adapted as necessary.
To enable a specified larger volume flow but at the same time to be able to control small volume flows as exactly as possible, one possibility is to produce or shift a control valve which has more than one, for example two working ranges, such as a lower volume flow range and one or more higher volume flow ranges.
From DE 103 33 236 A1 a hydraulic control system of the type mentioned is already known, which serves to control a cooling medium flow for cooling the clutch of a dual-clutch transmission in a motor vehicle. It comprises two control valves or two control pistons arranged in a housing, such that a radially outer control piston is guided to move axially in the housing and a second control piston moves in the first control piston. The two control pistons are each acted upon by the same pilot control pressure in opposition to the force of respectively associated restoring springs, the restoring springs having different spring constants so that the two control pistons are activated at different pilot control pressures. Since oil is supplied to and drained away from the inner control piston via ducts formed in the outer control piston, the two control valves are not independent, i.e. the emerging volume flow depends in each case on the respective relative and absolute position of the two control pistons. As a whole, the component concerned is of extremely complex design, difficult to produce, and therefore expensive. Furthermore, if one of the two control pistons fails, the entire control system breaks down and has to be replaced.